To meet the increasingly stringent ultra-low NOX emission requirements in the cement industry (<50mg/Nm3), this paper takes a 5 000t/d cement production line as an example to introduce the principle and engineering application of calciner deep self-denitrification technology. This technology extends the residence time of NOX in the reduction zone, strengthens the control of the reducing atmosphere, improves the system's denitrification efficiency by raising the height of TAD inlet, expanding the volume of the reduction zone, optimizing the layout of the denitrification duct, and implementing a “separated air, separated coal, separated material' system design. After the retrofit, the NO_X concentration at the outlet of the calciner system was reduced to 350mg/Nm3, ammonia consumption decreased to 2kg/t.cl, and standard coal consumption was reduced by 2kg/t.cl while maintaining the same clinker production, achieving both ultra-low NO_X emissions and optimized operating cost.